4.1.6.1 Overview
The antenna-tuning feature allows the use of:
- Parts with higher tolerance for the capacitors of the magnetic antenna loop, because doing so compensates for unintended shifts in the center frequency of the loop antenna. This is especially important because of the critical tolerance of C21, as shown in the following figure.
- Loop antennas with a greater quality factor, and, thus, less antenna loss
The preceding figure shows the principle operation of the antenna tuning integrated into the ATA8510/15. This feature is intended for use together with magnetic loop antennas at moderate power levels normally used in key fobs.
A TXMode impedance transformation network consisting of L3 and C9 matches the 50Ω load impedance from the loop antenna to the optimum load impedance needed for the power amplifier including the parasitics of the SPDT RF switch (see Impedance Matching of RFIN and RF_OUT).
An RXMode impedance transformation network consisting of L1(L2), C4(C3) transforms the input impedance of RFIN_LB(HB) to 50Ω seen on SPDT_ANT (see Impedance Matching of RFIN and RF_OUT).
The antenna tuning integrated into the ATA8510/15 is capable of tuning the resonance frequency of the loop antenna with small influence to impedance transformation by adjusting the internal tuning capacitance CTUNE at the end of the antenna. The antenna matching must consider a maximum RF voltage of 3V peak at the ANT_TUNE pin (see parameter no. 12.30 in RF Transmit Characteristics). Finally, the absolute maximum rating for this pin is 4V peak (see Absolute Maximum Ratings).
Automatic antenna tuning takes place in TXMode, but the result can be used for RXMode too. This is possible because the bandwidth of a loop antenna in receive mode is wider than in transmit mode due to the fact that the input impedance of the receiver transformed into the loop antenna reduces the quality factor. The receive bandwidth is, thus, wider than the transmit bandwidth and the result from TXMode antenna tuning is usable for RXMode.